This invention relates to nonwoven fabrics suitable for use in the electrical insulation field, and to a process for preparing such fabrics.
In the insulation of electrical equipment, as for example in the armature winding, slot lining of generators, the formation of circuit boards, and the like, maintenance of continuity of insulating value is of prime importance. The primary insulating materials, such as varnishes, epoxy resins, or mica, and the like, do not form self-sustaining manipulatable sheets of enough strength to allow their application to electrical components. Such materials therefore are commonly mounted on a supportive base such as glass cloth, nonwoven fabrics, special papers, and the like.
Supportive bases of this type must possess, as a primary requisite, a high degree of resistance to thermal degradation and to elongation or deformation under the stresses developed during the application of the insulating material and during the use of the equipment, which customarily involve elevated temperatures. If the base material stretches or deforms under low or moderate stresses, the non-elastic varnish or resin coating may develop cracks, leading to electrical leakage and loss of insulating value.
This is especially true when the primary insulating material is mica, highly desired because of its unique combination of electrical, thermal, and mechanical properties. One form of primary insulation is a mica paper, made of natural mica comminuted into finely-divided form and reconstituted into a sheet of overlapping, horizontally stratified platelets. The overlapping mica platelets form an excellent insulating medium so long as their overlapping relationship is maintained. Such sheets, however, have little strength, and are generally mounted on a strong supportive base by means of a resin, such as an epoxy, securing the mica sheet to a glass cloth.
Similar considerations govern the case where a strip of plastic film, such as polyethylene terephthalate film, is the primary insulating medium. The desirable electrical properties of such films are unfortunately coupled with a low tear strength: for many applications, therefore, it is common practise to form an adhesive laminate of a film with a nonwoven fabric, the fibrous nature of the latter serving to enhance the tear strength of the film.
With the constant demand for more compact electrical components, there is a concomitant demand for fabrics which can be processed into insulating materials of high efficiency and decreased thickness. The fabrication of ultra-thin glass fabrics is expensive and cumbersome, so that on a practical basis, nonwoven fabrics are becoming more and more widely used in the development of thin, economical, base supports for layers of electrical insulation. Heretofore, however, nonwoven fabrics with the necessary electrical properties, such as bonded nonwoven fabrics composed of polyester fibers bonded by the use of other polyester fibers of a lower softening temperature, have not offered a high degree of crosswise strength per unit of weight or thickness. In order to attain the desired value of crosswise tensile strength, the weight, and consequently the thickness, of the nonwoven fabric had to be increased to an undesirable degree.
In addition to thinness, however, the nonwoven fabric must, for reasons set forth above, possess what may be called high thermal stability: that is, a high degree of resistance to degradation and loss of strength at elevated temperatures, together with dimensional stability. Since for economic reasons it is expedient to form nonwoven fabrics from carded fibrous webs, wherein the fibers are oriented predominately in the machine direction, what is desired is a high value of crosswise tensile strength per unit of thickness.